Bacterial Leaf Spot

Xanthomonas campestris pv. alfalfae (syn. X. alfalfae and Phytomonas alfalfae) survives in infected residue that has been incorporated into soil or is lying on the soil surface, in hay, and in debris associated with seed. During warm, wet weather, bacteria are splashed or blown onto leaves and enter them through small wounds that have been made by any means. During dry weather, bacteria may enter leaves through wounds made by windblown soil particles. The bacteria multiply inside the leaf and frequently ooze to the leaf surface, where they may be splashed by rain or rubbed by leaf to leaf contact onto adjacent healthy leaves.

Distribution.

Wherever alfalfa is grown under warm, wet conditions.

Symptoms.

Diseased seedlings are often killed or stunted at high temperatures. Initially, small, water soaked spots occur in chlorotic areas on leaflets. The spots enlarge to irregular shaped

lesions (23 mm in diameter) with chlorotic margins that are pronounced on the underside of leaflets. Eventually, the lesions become light yellow to tan, often with a lighter center, and have a translucent, papery texture. Lesions often glisten because of the dried bacterial exudate on their surface. Diseased leaves defoliate prematurely. Stem lesions are water soaked initially, then turn brown or black.

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